1. Field of the Invention
The invention relates to an arrangement for controlling the amplitude and the phase of frequency components of an audio signal which is available in digital form, the frequency components being located in a predetermined frequency band. Such an arrangement is suitable for use as a bass control circuit, as a treble control circuit, or as an equalizing circuit, and will be denoted a digital tone control arrangement hereinafter.
2. Description of the Prior Art
In audio apparatus generally, an analogue audio signal is processed whose frequency spectrum is located in the audio frequency band from approximately 0 Hz to approximately 20 kHz and which is converted into an acoustic signal via a loudspeaker. The energy in this acoustic signal is, however, highly dependent on the acoustic impedance of the enclosure in which the loudspeaker is accommodated. As this impedance has a frequency-dependent behaviour, the energy in the acoustic signal depends on the amplitude and the phase of the different frequency components of the analogue audio signal.
In order to prevent certain frequency components in the audio signal from making too small a contribution to the acoustic signal, this analogue audio signal is applied to the loudspeaker via a tone-control arrangement. This tone control arrangement may be of such a construction that therein the amplitude and phase of those frequency components of the analogue audio signal which are located in the frequency band from for example, 0 Hz to approximately 300 Hz can be reinforced or attenuated with respect to the remaining components. This is called bass control. Alternatively, this tone control arrangement may be of such a construction that therein the amplitude and the phase of those frequency components of the analogue audio signal which are located in the frequency band from for example, approximately 300 Hz to 20 kHz can be reinforced or attenuated with respect to the remaining components. This alternative is called treble control. Also in contemporary audio apparatus tone control arrangements are used which are of such a construction that therein the amplitude and phase of those frequency components of the analogue audio signal which are located within a particular frequency band of a given width within the audio frequency band are reinforced or attenuated with respect to the remaining components; this is called spectrum equalization. Typically, to equalize the frequency spectrum, the overall audio frequency band is divided into a plurality of consecutive sub-bands each having a width of, for example, approximately 1 kHz and a tone control arrangement is then provided for each sub-band.
In known tone control arrangements which are arranged for processing an analogue audio signal, the desired reinforcing or attenuation of frequency components is realized by changing the resistance value of a resistance network. Thus, such a tone control arrangement has a transfer characteristic which depends on the resistance value of this variable resistance network. Hereinafter, this will be expressed by assuming that a family of transfer characteristics is associated with the tone control arrangement.
In the past few years the interest in the digitalization of analogue signals has generally greatly increased. This interest has also penetrated into the audio field. Already some years ago, efforts were successfully made to digitize an analogue audio signal, more specifically a musical signal, and to record it in digital form on a magnetic tape and even on a record. By means of a digital read arrangement the information present on the tape or on the record is converted into a digital audio signal which can be converted into the original analogue audio signal by means of a digital-to-analogue converter. A tone control can be applied in the above-described manner to this audio signal.
Each of the references 1 and 2 describes a digital tone control arrangement with which the desired tone control is obtained by performing processing operations on the digital audio signal. This arrangement comprises a recursive digital filter to which the digital audio signal is applied and which incorporates a recursive portion and a non-recursive portion. General information on digital filters and digital signal processing can be found in Reference 3, while the terminology relating to digital signal processing is contained in Reference 4. As is, for example, described in these References 3 and 4, both the recursive portion and the non-recursive portion of a recursive digital filter comprises a plurality of delay elements and a plurality of multipliers.
A main group of filter coefficients is associated with the recursive digital filter, that is to say a multiplying factor, alternatively denoted a filter coefficient, is applied to each multiplier. Together, these multiplying factors form this main group. Such a main group can be split into a first and a second subgroup, which, as is indicated in the References 1 and 2 must each comprise the same number of filter coefficients. The coefficients of the first subgroup are, for example, applied to the multipliers in the non-recursive portion and the coefficients of the second subgroup are applied to the multipliers of the recursive portion. Thus, the first subgroup determines the location of the zeros of the transfer function of the digital filter and the second subgroup then determines the locations of the poles of these transfer functions (see also Reference 3, chapter 4). When the location of the poles and the zeros of the transfer function are known, the transfer characteristic of the recursive digital filter and consequently also of the tone control arrangement are then fully known. As a family of transfer characteristics must be realized with the digital tone control arrangement, as also with its analogue counterpart, a main group of filter coefficients must be available for each transfer characteristic. It is then customary to use in the tone control arrangement a memory means having a number of addressable memory fields. A main group of filter coefficients is stored in each memory field. More specifically, each memory field has a number of memory locations which are each arranged to store a filter coefficient of the relevant main group. The required addresses for the addressable memory fields are generated by an address generator which can be operated by a user of the audio apparatus by means of control elements.
From the foregoing, it will be obvious that the number of memory locations required is directly proportional to the number of filter coefficients in a main group and to the number of transfer characteristics to be realized.